1. Field of the Invention
The present invention relates to a copier, printer, facsimile apparatus or similar image forming apparatus. More particularly, the present invention relates to an image transferring and recording medium conveying device for use in an image forming apparatus and including a belt configured to form a nip between it and an image carrier for transferring a toner image from the image carrier to a sheet or similar recording medium, bias applying means for applying a bias for image transfer to the belt, and control means for controlling the bias applying means.
2. Description of the Background Art
It is a common practice with an image forming apparatus to form a toner image on a photoconductive drum or similar image carrier and then transfer the toner image to a sheet being conveyed by a belt via a nip between the drum and the belt. The belt has medium electric resistance. To transfer the toner image from the drum to the sheet, a transfer bias source applies a charge opposite in polarity to toner to the belt. The sheet with the toner image is conveyed to fixing means by the belt while being electrostatically retained on the belt.
The conveyance of the sheet by the belt is based on the following principle. Electrostatic adhesion acts between the belt and the sheet due to an image transfer charge when the toner image is transferred from the drum to the sheet at the nip. When this electrostatic adhesion and the tendency of the sheet to straighten (hardness hereinafter) overcome adhesion acting between the sheet and the drum also derived from the image transfer charge, the sheet is separated from the drum and electrostatically retained on the belt.
However, if charge injection from the surface of the belt having medium resistance into the sheet is excessive, then the belt and sheet repulse each other because they are of the same polarity, weakening the adhesion. Further, when the sheet is curled, particularly curled in the direction in which the sheet tends to wrap around the drum (face curl), the hardness of the sheet tends to obstruct separation. Moreover, when the sheet not closely contacting the belt due to such the face curl enters the nip, spatial discharge occurs in the gap between the sheet and the belt due to the bias before the former is brought into close contact with the latter by the pressure of the nip. The spatial charge charges the sheet to the same polarity as the transfer charge, also weakening the adhesion between the sheet and the belt. Particularly, when the adhesion between the leading edge portion of the sheet in the direction of conveyance and the belt is weakened, the leading edge of the sheet is apt to wrap around the drum without being retained on the belt, resulting in defective sheet separation. The separation of the sheet from the drum is susceptible not only to the resistance of the belt but also to the resistance, thickness and smoothness of the sheet. A thin sheet and a sheet with low resistance, among others, are disadvantageous as to separation from the drum.
Technologies relating to the present invention are disclosed in, e.g., Japanese Patent Laid-Open Publication No. 2001-154505.